A synchronizer clutch which is employed in a vehicle change-speed transmission for a gear-coupling purpose is often composed of a cone synchronizer clutch of Borge Warner-type having a synchronizer key or a variation of Borg Warner-type having a synchronizer pin. In each of these cone-type synchronizer clutches, a synchronous rotation is attained by a frictional engagement between cones of the driving and driven sides of the clutch. Because cone diameter can be made larger in a pin-type clutch than in a key-type clutch owing to the arrangement of splines, pin-type synchronizer clutches are preferred in heavy-duty working vehicles such as tractors. At any events, a cone synchronizer clutch is associated with each of the change gear trains in a vehicle multi-ratio transmission so as to achieve respective change ratios by a selective operation of the respective synchronizer clutches as shown, for example, in JP, A No. 2-212225.
Frictional engaging area provided by a pair of cones in a cone synchronizer clutch is limited even in a pin-type clutch which can be designed to have a relatively large capacity. As a result of this, it has been experienced that cone surfaces in a cone synchronizer clutch which is associated with a heavily operated change-speed gear train such as a frequently actuated gear train or a gear train for a low speed, high torque change transmission cannot sometimes absorb a sufficient amount of energy so that seizing of such cone surfaces is caused. From this, it is considered to employ multidisc-type synchronizer clutches already known from, for example, U.S. Pat. No. 4,830,159 and No. 5,058,455, so as to enlarge frictional engaging area and to thereby eliminate such problem. A synchronizer clutch of this type is, however, relatively expensive because of its large number of elements. Further, a multidisc synchronizer clutch has a problem that some engagement between its frictional discs or elements of the driving side and of the driven side may be caused in a clutch-disengaged condition due to rotational torque of the frictional elements of driving side. Such engagement of clutch may cause an unexpected start of the vehicle at a neutral condition of the change transmission.
Accordingly, a primary object of the present invention is to provide an improved multi-ratio transmission which eliminates the drawback of a change transmission employing cone-type synchronizer clutches and which also solves the problems of employing multidisc-type synchronizer clutches.